Positive ionic drift in T2K gas Final report Fred Hartjes NIKHEF Nikhef/Bonn LepCol meeting February 24, 2020 ‹#› Nikhef/Bonn LepCol meeting, February 24, 2020 1
Setup for measuring positive ion drift Digital ■ Drift cathode used as an antenna oscilloscope -HV ■ Ions from the laser beam Drift cathode instantaneously induce charge on drift cathode ■ Ions move towards the drift cathode, generating an induction 40.0 mm ~30 mm current ■ Current terminated at arrival at the Laser drift cathode avalanche ■ In addition ions leaking through the grid during the avalanche induce charge as well 8 quad testbox ■ Measurements triggered by laser diode ■ On the scope averaged over 32 triggers Fred Hartjes Nikhef/Bonn LepCol meeting, February 24, 2020 2
Setup of the charge signal Instabilities/ noise reduced/cancelled collection ■ Filters on control line and output of drift field HV supply Oscilloscope -HV AD549J ■ LabVIEW communication with HV - 250 M supplies had to be stopped during data 1n 3 kV taking + Drift cathode 250 M ■ Remaining micro discharges at drift 90p cathode ■ Reducing 50 Hz pickup Best scope sensitivity 1 mV/div • Signal was often clipped at 0.5 mV/div ■ Laser instability 20 – 30% rms Averaging over 32 laser shots Fred Hartjes Nikhef/Bonn LepCol meeting, February 24, 2020 3
Simplified electronic circuit ■ Circuit values measured with test pulse ■ Through 1 pF and 100 MOhm Oscilloscope ■ Parasitic capacity of drift cathode, Lemo cable, electronics measured as 1n 89.5 pF ■ RC time 10 +/- 0.5 ms Drift cathode 250 M 250 M ■ We get an ideal integrator curve by 90p deconvolution of the measured curve from the RC time constant ■ Also taking into account the voltage change on 1 nF coupling capacitor Fred Hartjes Nikhef/Bonn LepCol meeting, February 24, 2020 4
■ Vgrid = -150V => no gas gain Primary ionization by laser ■ Ionizing exclusively TMPD ( N, N, N’, N’ - Tetramethyl-1,4-phenylendiamin) Vgrid = -150 V ■ In the chamber gas as a pollution in the ppb level Field: 280 V/cm ■ Laser beam at about 30 mm from the drift cathode 14-2-2020 Deconvoluted curve ■ Note the sharp bend when the ions are collected by the drift cathode ■ Initial drift time 8 ms ■ => velocity ~3.8 m/s ■ But possible tail of slow ions Initially created Measured ■ Phenomenon hard to measure because of instabilities ion cloud Average over 32 on the charge signal laser shots ■ Measurement with blocked laser has been subtracted ■ Integrated charge 200 - 260 fC ■ => 1.25 - 1.6 M ions ■ Above the 8 grids: 129 – 167 fC ■ => ~ 6.7 – 8.8 electrons entering each hole Fred Hartjes Nikhef/Bonn LepCol meeting, February 24, 2020 5
Ion measurement at working point Vgrid = -340 V Field: 280 V/cm ■ Mainly ions leaking through 14-2-2020 the grid ■ Measurement is polluted by two phenomena Primary ionization ■ Signal of the ions from the primary ionization ■ Cross talk from the grids onto the drift cathode Fred Hartjes Nikhef/Bonn LepCol meeting, February 24, 2020 6
Primary ionization subtracted Vgrid = -340 V Field: 280 V/cm 14-2-2020 ■ Sudden charge jump at laser firing ■ Ions starting drifting from the grid should not induce an immediate charge ■ Unexplained charge jump of Unexplained ~ 350 fC charge jump Fred Hartjes Nikhef/Bonn LepCol meeting, February 24, 2020 7
Test with extremely low drift field Vgrid = -340 V Field: 15 V/cm 14-2-2020 ■ Drift field 15V/cm ■ Primary ionization subtracted ■ Sudden jump of the integrated charge curve mostly cancelled after 5 ms ■ Time constant does not correspond to the system time constant (10 ms) ■ Source of the rapid charge rise: cross talk by the eight grids under the laser beam to the drift cathode ■ After that slow rise due to the slow ionic drift ■ Small part of the initial peak charge remains ■ ~ 75 fC (1 fC = 6242 e-) Fred Hartjes Nikhef/Bonn LepCol meeting, February 24, 2020 8
Time constant of the charge jump ■ T RC = 1.33 ms ■ Each of the 8 involved grids has a supply resistor of 100M Ω ■ => C grid = 13.3 pF for a single grid Fred Hartjes Nikhef/Bonn LepCol meeting, February 24, 2020 9
Low field measurement with cross talk compensated Vgrid = -340 V Field: 15 V/cm ■ Current peak at the beginning remains 14-2-2020 ■ Cannot be compensated by cross talk correction ■ Here ~ 60 fC (1 fC = 6242 e-) 15 V/cm ■ Peak is 4 ms wide, NOT a delta function ■ Peter effect?? Induced current from differentiation of deconvolution curve. dT = 2 ms Fred Hartjes Nikhef/Bonn LepCol meeting, February 24, 2020 10
Induced ionic current at working point ■ Current corrected for grids cross talk ■ TRC = 1.5 ms ■ Same peak as for the low field dT = 1 ms measurement Vgrid = -340 V ■ It has a duration of ~ 1.5 ms Field: 280 V/cm 14-2-2020 280 V/cm Fred Hartjes Nikhef/Bonn LepCol meeting, February 24, 2020 11
How big is the voltage jump on the grids? ■ The grid supply current induced by the laser beam can be measured ■ 0.48 nA ■ Laser frequency: 2.68 Hz ■ => 180 pC per laser pulse ■ Total grid capacity: 8 x 13.3 = 106 pF ■ => voltage jump on grids 1.7 V ■ Induced charge: 350 fC ■ => parasitic capacity between 8 grids and drift cathode: 0.2 pF Fred Hartjes Nikhef/Bonn LepCol meeting, February 25, 2020 12
Ionic current at work point ■ Tail of slow ions with drift times between 15 and 30 ms Vgrid = -340 V ■ From deconvolution using TRC = Field: 280 V/cm 14-2-2020 10 ms ■ System time constant measured using test pulses through 1 pF capacitor and 100 M Ω resistor dT = 1 ms 280 V/cm TRC = 10 ms Fred Hartjes Nikhef/Bonn LepCol meeting, February 24, 2020 13
Deconvolution for other time constant ■ Tail of slow ions has disappeared using TRC = 12.5 ms Vgrid = -340 V ■ So are these slow ions really there or Field: 280 V/cm has the system time constant been 14-2-2020 wrongly measured? dT = 1 ms 280 V/cm TRC = 12.5 ms Fred Hartjes Nikhef/Bonn LepCol meeting, February 24, 2020 14
Grid leakage and gas gain ■ Total avalanche charge per laser shot = 180 pC ■ Induced charge on drift cathode = 2.8 pC ■ Subtract primary ionization (0.2 pC) ■ => Ion leakage through grid at working point (280 V/cm, -340 V grid): 1.45% ■ => Gas gain 1075 – 1400 @ Vgrid = -340 V ■ Using only the primary charge above the grids ■ 129 -167 fC ■ Possible saturation effects (7 – 9 electrons entering each hole during ~ 100 ns) ■ Ionic drift time over 50 μ m at -340 V grid is 50 ns ■ So fresh electrons entering the gap when still positive ions from the previous avalanche are present Fred Hartjes Nikhef/Bonn LepCol meeting, February 24, 2020 15
Leakage for different drift fields ■ Leakage rising with field strength from <1.0 to 1.7 % ■ Accuracy affected by instabilities ■ The 1.45% value was measured a few hours before the other measurements ■ The fit suggests rather 1.3% Fred Hartjes Nikhef/Bonn LepCol meeting, February 24, 2020 16
Comparing leakage fraction with Chefdeville ■ For our working point (280 V/cm, -340 V grid and 50 μ m grid gap) => Field ratio = 252 ■ Chefdeville => leakage ~ 2.2 % ■ This measurement: 1.3 – 1.45% Fred Hartjes Nikhef/Bonn LepCol meeting, February 24, 2020 17
Ar+ drift velocity vs drift field ■ Not valid for low fields ■ Curve does not pass X, Y = 0, 0 ■ Linear fit may not be correct ■ Mobility gets smaller at low fields Fred Hartjes Nikhef/Bonn LepCol meeting, February 24, 2020 18
Ionic mobility vs drift field ■ Assuming the main peak originates from Ar+ ions ■ Literature (Ar+ ions in Ar) (Madson, Hornstein 1967, 1951) ~ 1.3 – 1.5 cm 2 V -1 s -1 measured in ■ 20 – 25 kV/cm range ■ For mobility = 1.5 we get for the ionic drift time across the amplification gap of 50 um: ~ 50 ns Fred Hartjes Nikhef/Bonn LepCol meeting, February 24, 2020 19
Summary positive ion measurements ■ 8-quad testbox has not been designed for ionic measurements ■ Additional analysis is required to extract the physical phenomena ■ Using the drift cathode as an antenna the leakage current of positive ions through the grid could well be measured ■ Disturbing effects from electronic noise, cross talk from the grids, laser instability could be minimized by shielding, filtering, offline compensation and averaging ■ The leakage fraction of ions through the grid at the working point was measured considerably lower than earlier (Chefdeville) ■ Some additional ions (0.11% of the avalanche charge) generated during the first few ms ■ Peter effect? Fred Hartjes Nikhef/Bonn LepCol meeting, February 24, 2020 20
Summary positive ion measurements cntd ■ Uncertainty of time constant of the measuring system ■ 10 ms measured with test pulses => reliable ■ 12.5 ms removes tail of slow ions ■ Are the slow ions there or not?? ■ Measured gas gain at -340 V grid possibly too small (1075 - 1400) ■ Possible saturation effects because of the extremely high primary ionization ■ Electrons entering the amplification gap when positive ions are still present ■ To be verified with ToT measurement ■ Unattenuated laser pulse gives significant voltage drop on the grids (1.7 V) ■ => Cross talk to drift cathode Fred Hartjes Nikhef/Bonn LepCol meeting, February 24, 2020 21
Recommend
More recommend
